clang tools: Add --bootstrap to the documentation for update.py

docs/clang_tool_refactoring.md

 # Clang Tool Refactoring
 
 [TOC]
 
 ## Introduction
 
 Clang tools can help with global refactorings of Chromium code. Clang tools can
 take advantage of clang's AST to perform refactorings that would be impossible
 with a traditional find-and-replace regexp:
 
@@ skipping 79 matching lines @@
 
 *   Clang tools run actions serially, so runtime scales poorly to tens of
     thousands of files.
 *   A parsing error in any file (quite common in NaCl source) prevents any of
     the generated replacements from being applied.
 
 ## Building
 Synopsis:
 
 ```shell
-tools/clang/scripts/update.py --force-local-build --without-android \
+tools/clang/scripts/update.py --bootstrap --force-local-build --without-android \
   --tools blink_gc_plugin plugins rewrite_to_chrome_style
 ```
 
 Running this command builds the [Oilpan plugin](https://chromium.googlesource.com/chromium/src/+/master/tools/clang/blink_gc_plugin/),
 the [Chrome style
 plugin](https://chromium.googlesource.com/chromium/src/+/master/tools/clang/plugins/),
 and the [Blink to Chrome style rewriter](https://chromium.googlesource.com/chromium/src/+/master/tools/clang/rewrite_to_chrome_style/). Additional arguments to `--tools` should be the name of
 subdirectories in
 [//tools/clang](https://chromium.googlesource.com/chromium/src/+/master/tools/clang).
 Generally, `--tools` should always include `blink_gc_plugin` and `plugins`: otherwise, Chromium won't build.
 
+It is important to use --bootstrap as there appear to be [bugs](https://crbug.com/580745)
+in the clang library this script produces if you build it with gcc, which is the default.
+
 ## Running
 First, build all chromium targets to avoid failures due to missing dependecies
 that are generated as part of the build:
 
 ```shell
 ninja -C out/Debug
 ```
 
 Then run the actual tool:
 
@@ skipping 48 matching lines @@
 
 ```shell
 test_tool.py <tool name>
 ```
 
 The name of the tool binary and the subdirectory for the tool in
 `//tools/clang` must match. The test runner finds all files that match the
 pattern `//tools/clang/<tool name>/tests/*-original.cc`, runs the tool across
 those files, and compared it to the `*-expected.cc` version. If there is a
 mismatch, the result is saved in `*-actual.cc`.